How can an artificial general intelligence determine which information is true?
Not possible without some big restrictions. What it can do is look at known "good" sites and compare news with site that is potentially "bad". Obvious problem here is defining some sites as absolute truth. For example it can recognize, while reading text, that some politician said something. These sentences can be compared with other sites, and if there is significant difference, that news is candidate for false news.

In practical terms, program would extract sentences "i like cats", "says he likes cats", "cats that John likes" etc. We need part that recognizes something as a quote, part that extracts it and finally parser so we end up with structure stored in some form that contains meaning of sentence (john-like-cats). Also it can keep information of time and context in which it was said, like timestamp of an article, some proper nouns that can indicate place (XY conference, London...). Now, suspicious article can be compared and checked if it matches time, place, some context and contains quote that is similar. Finally it needs to compare how different it is from other quotes. "...hates cats" should be labeled as potential fake news, but "likes dogs", "thinks cats are OK", "sings well" etc. should not. This can be expanded into comparison of whole articles.

There are many features that can be used to define particular article as fake. Interesting feature  for finding fake sites could be bias when it comes to particular (political, economical, ecological...) opinion. But in the end machine can't decide if the article is fake without comparing it to other articles. It is bound to closed system that reflects real world in subjective way.